In most places it doesn't make a difference to the outcome of the legal process what it does do is give you a quicker simpler off ramp from the legal process (which reduces costs) and may stop some idiots even trying to sue in the first place.

"Do not iron clothes while on body" should not be required to not be found liable, but it does change the question in court from providing discovery for safety consideration, how comprehensive is the manual, how... and the costs involved with that to "Did the customer use the device in a way that was it was clearly labelled to not be used? Did any part of the product packaging or instructions contradict this warning? ...Dismissed".

On top of that, I think my canister of Lysol wet wipes and many other bottles of cleaning chemicals says something like "it is against federal law to use this product for any purposes other than its intended use"

Like, yeah it's illegal to do illegal stuff with or without the label, but at least Lysol could say "we did tell him that he can't use it for that."